Ram-based positioning system

ABSTRACT

One aspect of the invention provides a positioning system comprising at least one master ram and respective slave ram, at least one travel stop and at least one pressure relief valve. The or each travel stop is arranged to restrict the travel of one or more slave piston when the system is misaligned, at least one pressure relief valve being arranged to vent excess pressure in the operating fluid arising as a result of such restriction. The positioning system is particularly suited for incorporation into a steering system, especially for an articulated vehicle.

FIELD OF THE INVENTION

The present invention relates to a ram-based positioning system. Theinvention relates particularly, but not exclusively, to a steeringsystem for an articulated vehicle.

BACKGROUND TO THE INVENTION

It is known for the steering system of an articulated vehicle tocomprise a ram-based positioning system. Typically, the steering systemcomprises one or more pairs of master/slave hydraulic rams wherein theslave rams are arranged to steer the rear axle of the vehicle. A problemwhich such systems is that they can often become misaligned as a resultof, for example, loss of hydraulic liquid through leakage or through apressure relief valve during an emergency operation.

In a conventional system, alignment has to be restored manually,typically by centralising the steering axle and king pin sensor to areference point, then opening and closing the system valves in sequenceand manually re-priming the system. This is a relatively time consumingand awkward procedure, not least because the driver needs to leave thevehicle unless he has assistance.

It would be desirable, therefore, to provide a ram-based positioningsystem which may readily be re-aligned by, for example, the driver of anarticulated vehicle, without the need for assistance or the need toleave the vehicle.

SUMMARY OF THE INVENTION

Accordingly, a first aspect of the invention provides a positioningsystem comprising at least one master ram and respective slave ram, eachram being fluid operable and having a respective piston and a respectivepiston chamber, the or each master ram and respective slave ram beingarranged, when in a state of normal alignment, so that actuation of themaster piston with respect to the master piston chamber causes acorresponding actuation of the slave piston with respect to the slavepiston chamber, wherein the system further includes at least one travelstop and at least one pressure relief valve, the or each travel stopbeing arranged to restrict the travel of one or more slave piston whenthe system is misaligned, at least one pressure relief valve beingarranged to vent excess pressure in the operating fluid arising as aresult of such restriction.

By venting the excess fluid pressure in this way, the system is broughtback into normal alignment.

Preferably, in the state of normal alignment, the or each master andrespective slave pistons are actuatable between a respective fullyextended state and a respective fully retracted state, wherein the oreach travel stop is arranged to prevent one or more slave piston fromreaching at least one of said fully extended or fully retracted stateswhen the system is misaligned. Once the slave piston is prevented frommoving further, excess fluid pressure arises as the master pistoncontinues to move to its fully extended or fully retracted state.

Preferably, each ram comprises a first port and a second port for theinlet and outlet of operating fluid, wherein the ports of the master ramare connected to the ports of the slave ram with a one-to-onecorrespondence, and wherein a respective pressure relief valve isconnected to at least one pair of connected ports. More preferably, arespective pressure relief valve is connected to each pair of connectedports.

Preferably, the, or each, pressure relief valve is arranged to directvented operating fluid to an accumulator.

A second aspect of the invention provides a steering system comprisingthe positioning system of the first aspect of the invention.

Preferably, the or each master ram is mountable on a vehicle such that,upon turning of the vehicle, the or each master piston is actuated, andwherein the or each respective slave piston is adapted for steering atleast one wheel of the vehicle. More preferably, said at least one wheelis carried by a steerable axle, the or each respective slave pistonbeing adapted to steer said steerable axle.

Preferably, the steering system is arranged for steering an articulatedvehicle having a tractor unit pivotably coupled to a trailer unit,wherein the or each master piston is actuated upon relative pivotingmovement between the tractor unit and the trailer. More preferably, thearrangement is such that, upon extension or retraction of the or eachmaster piston, the respective slave piston is retracted or extendedrespectively by a corresponding amount.

Preferably, the steering system comprises a first master ram and asecond master ram, each associated with a respective slave ram, therespective slave pistons being coupled to a steerable axle of thevehicle at locations on opposite sides of the pivoting axis of thesteerable axle, the arrangement being such that, upon turning of thevehicle, one of the first and second master pistons is caused toretract, the other being caused to extend, the corresponding actuationof the respective slave pistons causing the steerable axle to pivotabout its pivoting axis.

Preferably, the or each travel stop is provided on the chassis of avehicle on which, in use, the steering system in mounted and arranged torestrict the pivoting movement of the steerable axle. Alternatively,the, or each, travel stop is provided on a respective slave ram.

A third aspect of the invention provides a vehicle comprising thesteering system of the first aspect of the invention. The invention isparticularly suited for use with articulated vehicles.

Preferably, the rams are hydraulically operable, the preferred hydraulicoperating liquid being oil.

Other advantageous aspects of the invention will be apparent to thoseordinarily skilled in the art upon review of the following descriptionof a specific embodiment of the invention and with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A specific embodiment of the invention is now described by way ofexample and with reference to the accompanying drawings in which likenumerals are used to indicate like parts and in which:

FIG. 1 is a plan view of an articulated vehicle including a preferredram-based steering system embodying one aspect of the invention;

FIG. 2 is a schematic view of a preferred ram-based positioning systemembodying another aspect of the invention, and being suitable for use inthe steering system in FIG. 1;

FIG. 3 is a schematic view of a hydraulic circuit for use with thepositioning system of FIG. 2;

FIG. 4 is a plan view of the articulated vehicle of FIG. 1, wherein thesteering system is in a partially locked state and the rear axle isproperly aligned;

FIG. 5 is a plan view of the articulated vehicle wherein the steeringsystem is in a fully locked state and the rear axle is properly aligned;

FIG. 6 is a plan view of the articulated wherein the steering mechanismis in a straight ahead state and the rear axle is misaligned;

FIG. 7 is a plan view of the articulated vehicle wherein the steeringsystem is in partially locked state and the rear axle is misaligned.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIG. 1 of the drawings there is shown, generallyindicated at 10, an articulated vehicle comprising a tractor unit, ortractor, 12 and a trailer 14. The trailer 14 comprises a chassis 16 onwhich there is pivotably mounted a steerable rear axle 18 carrying a setof wheels 20. Depending on the size of the trailer 14 it may include oneor more further axles and wheel sets. In the illustrated embodiment, thetrailer 14 includes a further fixed axle/wheel set 15. The vehicle 10comprises a steering system which includes a positioning systemembodying the invention. In the preferred embodiment, the positioningsystem comprises a first and a second slave ram 22, 24 and a first andsecond master ram 26, 28, connected to a hydraulic circuit (not shown inFIG. 1) which includes a valve circuit, or valve block 29. Each ram 22,24, 26, 28 preferably comprises a double acting ram and comprises arespective piston and piston chamber in normal manner.

The rear axle 18 is pivotable with respect to the chassis 16 by thefirst and second slave rams 22, 24, which have their respective pistonchambers mounted on the chassis 16 and their respective. pistons coupledto the rear axle 18. The first and second master rams 26, 28 are alsomounted on the chassis 16. The master rams 26, 28 are coupled to arespective slave ram 22, 24 so that operation of a master ram 26, 28causes a corresponding operation of the respective slave ram 22, 24.

The trailer 14 is pivotably coupled to the tractor 12 by conventionalcoupling, or linkage mechanism which, in the preferred embodiment,comprises a kingpin mechanism 30 (commonly referred to as a kingpinsteering mechanism). The linkage mechanism 30 also forms part of thesteering system of the vehicle 10 in that it transmits angular orpivotal movement of the tractor 12 with respect to the trailer 14 to themaster rams 26, 28. To this end, the kingpin mechanism 30 includes acrank 32 for converting rotary movement, imparted to the crank 32 duringuse by the kingpin mechanism 30, into reciprocal movement of the pistonsof the master rams 26, 28. In the preferred embodiment, the crank 32 issubstantially “T-shaped” (sometimes referred to as a bell crank, orunitary bell crank) and has one end coupled to the kingpin 34, the othertwo ends being connected to a respective piston of the master rams 26,28.

The steering system also includes a respective stop member, or travelstop 36, 38, associated with each slave ram 22, 24. The travel stops 36,38, are arranged for engagement with the steerable rear axle 18 and/orthe pistons of the slave rams 22, 24 in certain dispositions of the axle18/rams 22, 24 as is described in more detail below. In FIG. 1 thetravel stops 36, 38 are mounted on the chassis 16 but need notnecessarily be so—for example, the travel stops 36, 38 may alternativelybe incorporated into the slave rams 22, 24 themselves.

Referring now to FIG. 2, there is shown, generally indicated at 40, thepreferred embodiment of the ram-based positioning system. In thepreferred embodiment, each ram 22, 24, 26, 28 comprises a hydraulic ramcomprising a respective piston 42, 44, 46, 48 and respective pistonchamber in the form of cylinders 52, 54, 56, 58. Each cylinder 52, 54,56, 58 includes two respective ports A, B, for the input and output ofhydraulic operating liquid, typically oil. The arrangement is such thatinflow of hydraulic liquid via port A causes the respective pistons 42,44, 46, 48 to retract and an outflow of hydraulic liquid from therespective port B, while inflow of hydraulic liquid via port B causesthe respective pistons 42, 44, 46, 48 to extend and an outflow ofhydraulic liquid from the respective port A. Equally, a retraction ofthe respective pistons 42, 44, 46, 48 causes an outflow from therespective port B, while an extension of the pistons 42, 44, 46, 48causes an outflow from the respective port A.

The master rams 26, 28 are hydraulically connected to the respectiveslave rams 22, 24 such that the respective port As and port Bs areconnected together in a one-to-one correspondence. Thus, when the piston46, 48 of a master ram 26, 28 extends, the piston 42, 44 of therespective slave ram 22, 24 retracts. Similarly, when the piston 46, 48of a master ram 26, 28 retracts, the piston 42, 44 of the respectiveslave ram 22, 24 extends.

Before use, the system 40 is primed with hydraulic fluid in conventionalmanner such that the relative states (in terms of pistonextension/retraction) of the rams 22, 24, 26, 28 are properly aligned(normal alignment) for the intended use. Thus, for a givenextension/retraction of the master ram 26, 28, there is an appropriatecorresponding retraction/extension of the respective slave ram 22, 24.It is noted that in FIG. 2, all of the rams 22, 24, 26, 28 are shown ina retracted state, which is not a normal operating mode of the system 40when used to implement the steering system described above. It will alsobe noted that, although the present embodiment requires that therespective port As of the rams are connected together, as are therespective port Bs, this need not necessarily be the case. For example,in alternative applications, the arrangement may be such that anextension in one (or both) of the master pistons 46, 48 causes anextension in the respective slave piston 42, 44.

The system 40 may become misaligned, i.e. removed from the normalalignment state, for a variety of reasons including loss of hydraulicliquid through leakage that commonly occurs over a period of timethrough, for example, cylinder seals (not shown) and valve assemblies(not shown). Hydraulic liquid may also be lost during emergencyoperations in which hydraulic liquid is shed through relief valves. Whenmisaligned, the relative states of the rams 22, 24, 26, 28 no longerconform to the required relative arrangement and, as a result, thepositioning system 40 does not operate properly.

The valve block 29 containing a hydraulic valve circuit (described belowwith reference to FIG. 3) comprises four main hydraulic ports D, E, F, Gfor the input and output of hydraulic liquid. The rams 22, 24, 26, 28are connected to the valve block 29 by hydraulic lines (shown dashed inFIG. 2) such that port D of the valve block 29 is hydraulicallyconnected to the respective port A of rams 22, 26; port E ishydraulically connected to the respective port A of rams 24, 28; port Fis hydraulically connected to the respective port B of rams 22, 26; andport G is hydraulically connected to the respective port B of rams 24,28. The hydraulic circuit within the valve block 29 implements apressure relief system that enables the positioning system 40 to bereadily re-aligned, as is described below.

Referring now to FIG. 3, there is shown the hydraulic circuit, generallyindicated at 50, implemented by valve block 29. In addition to ports D,E, F and G, the circuit 50 includes a priming port P which isconnectable to a conventional priming pump (not shown) in conventionalmanner. A one-way valve, or check valve 62, is associated with thepriming port P to prevent hydraulic liquid from leaving the circuit 50by this port P. The circuit 50 further includes an accumulator port ACCwhich is connectable to a conventional hydraulic accumulator (not shown)in conventional manner. The priming pump and hydraulic accumulator areused to charge, or prime, the valve circuitry 50 and associatedhydraulic circuit in conventional manner.

The circuit 50 comprises a main hydraulic feed line 64 which is inhydraulic communication with each of ports D, E, F and G via arespective one-way valve 66, each valve 66 being arranged to allowhydraulic liquid to flow from the main feed line 64 and out through therespective port D, E, F, G, but not in the reverse direction.

Each port D, E, F, G is also in hydraulic communication with arespective conventional pressure relief valve 68 which, in the preferredembodiment, are adjustable (or variable) pressure relief valves. Eachpressure relief valve 68 is located between the respective port D, E, F,G and the main feed line 64 and is arranged to allow hydraulic liquid toflow from the respective port D, E, F, G to the main feed line 64 onlywhen the pressure of the hydraulic liquid exceeds a threshold value, thethreshold value being determined by the setting of the respectivevariable pressure relief valve 68. The pressure relief valves 68 do notallow hydraulic liquid to flow in the reverse direction.

Circuit 50 further includes two conventional flow control valves 70which connect, when open, ports D and G and ports F and E, respectively.In normal operation of the circuit 50, the valves 70 remain closed andare effectively open-circuit (i.e. isolating port D from port G and portE from port F). During the initial setting up of the position system 40,however, the flow control valves 70 are opened to facilitate initialalignment of the positioning system 40. In the context of the steeringsystem outlined in FIG. 1, this means that the hydraulic liquid is ableto flow directly between the rams on either side of the trailer 14. Thisallows the pistons 42, 44 of the slave rams 22, 24 to move independentlyof the pistons 46, 48 of the master rams 26, 28 so that the steeringsystem can be properly aligned during the setting up of the hydrauliccircuitry.

The operation of the positioning system 40 is now described in thecontext of a vehicle steering system. FIG. 1 shows the articulatedvehicle 10 in a straight ahead state in which the tractor 12 and thetrailer 14 are substantially in register with one another i.e. therespective longitudinal axis of the tractor 12 and trailer 14 aresubstantially coincident. In this state, when the positioning system 40is in normal alignment (as shown in FIG. 1), the slave rams 22, 24 holdthe rear axle 18 substantially perpendicularly with respect to thelongitudinal axis of the tractor 12/trailer 14. Thus, the wheels 20 aresubstantially parallel with the trailer 14 i.e. ready for straight aheadmovement of the vehicle 10. Assuming that the slave rams 22, 24 arepositioned substantially ih register with one another on the chassis 16(as shown in FIG. 1), the slave pistons 42, 44 are extended bysubstantially equal amounts with respect to their cylinder 52, 54.Similarly, assuming that the crank 32 is symmetrically located on thechassis 16 and that the master rams 26, 28 are substantially in register(as shown in FIG. 1), then the master pistons 46, 48 are extended bysubstantially equal amounts with respect to their cylinder 56, 58. FIG.1 therefore shows the positioning system in normal alignment.

In FIG. 4, the steering system of the vehicle 10 is in a partiallylocked state (i.e. not at full turn) such that the tractor 12 isangularly disposed with respect to the trailer 14 (i.e. the respectivelongitudinal axis of the tractor 12 and trailer 14 are not coincident).The angular displacement of the tractor 12 with respect to the trailer14 is translated into a corresponding angular displacement of the crank32 with respect to its FIG. 1 “straight ahead” position. As the crank 32rotates (anti-clockwise as viewed in FIG. 4) it causes the piston 46 ofmaster ram 26 to extend and the piston 48 of master ram 28 to retract.This causes corresponding retraction of the slave piston 42 andextension of the slave piston 44. As a result, the rear axle 18 ispivoted anti-clockwise (as viewed in FIG. 4). Thus, the wheels 20, whichare now angularly disposed with respect to the chassis 16, are in aposition to facilitate the vehicle 10 making a clockwise turn (as viewedin FIG. 4). FIG. 4 also shows the positioning system in normalalignment.

FIG. 5 shows the vehicle 10 with its steering system in a fully lockedstate and still in normal alignment. The situation is similar to that ofFIG. 4 except the crank 32 is now in its maximum angular dispositionwith respect to the chassis 16 (for a clockwise turn in normal use).Consequently, the master pistons 46, 48 are in extended and retractedstates respectively corresponding to this fully locked state, and theslave pistons 42, 44 are in their corresponding retracted and extendedstates respectively (these piston states may be regarded as the fullyextended or fully retracted states of the pistons since they define thelimit of the pistons travel during normal use, although these states maynot correspond to the physical limits of extension/retraction of whichthe respective pistons are capable). The travel stop 38 is arranged, orpositioned, so that in this fully locked state, the rear axle 18 justcomes into contact with, or falls just short of, the travel stop 38. Itmay be said that the rear axle 18 substantially abuts with the travelstop 38 such that travel of the slave piston 44 is substantiallyunrestricted by the travel stop 38. It will be appreciated that thetravel stop 38 may alternatively be arranged to lie in the path of thepiston 44 itself such that the piston substantially abuts with thetravel stop 38 in the fully locked state.

The other travel stop 36 is similarly arranged with respect to the otherslave piston 42. Thus, when the steering system of the vehicle 10 is inthe opposite fully locked state (for an anti-clockwise turn as viewed inFIG. 5—not illustrated), the rear axle 18 (or alternatively the slavepiston 42) substantially abuts with the travel stop 36 such that travelof the slave piston 42 is substantially unrestricted by the travel stop36.

The forgoing descriptions referring to FIGS. 1, 4 and 5 relate to normaloperation of the steering system, and therefore of the positioningsystem 40, in normal alignment. In such conditions, the valve circuit 50is inactive. Hydraulic liquid is prevented from entering the circuit 50via ports D, E, F and G because the hydraulic pressure is not greatenough to allow the liquid to pass through the pressure relief valves68.

FIG. 6 shows the vehicle 10 in the straight ahead state but with thepositioning system 40 misaligned. The relative extension/retraction ofthe slave pistons 42, 44 do not correspond correctly with the respectivemaster piston 46, 48 extension/retraction in this state. As a result,the wheels 20 are angularly disposed with respect to the chassis 16 whenthey should be substantially parallel with it. Clearly, the orientationof the wheels 20 is unsuitable for straight ahead travel of the vehicle10 and requires re-alignment. Re-alignment is readily achieved using thepositioning system 40, and in particular the valve block 29, as is nowdescribed.

Starting from the position illustrated in FIG. 6, as the vehicle 10makes a clockwise turn, the slave piston 44 extends and the other slavepiston 42 retracts. However, because the slave piston 44 was alreadyfurther extended than it should have been before the vehicle 10 startedto turn (i.e. out of alignment), the rear axle 18 abuts with the travelstop 38 before the steering system of the vehicle 10 has reached thefully locked state (illustrated in FIG. 7). The travel stop 38 thusprevents the slave piston 44 from reaching the extended state thatcorresponds to the fully locked state shown in FIG. 5. However, as thesteering system continues towards the fully locked state, the masterpiston 48 continues to retract as normal. Because the slave piston 44 isprevented by the travel stop 38 from extending further, this results ina build up of hydraulic pressure since master piston 48 is forcinghydraulic liquid out of port B of master ram 28 but port B of slave ram24 is unable to receive this hydraulic liquid. The build up of hydraulicpressure causes hydraulic liquid to enter the valve block 29 via port Gand flow through the associated pressure relief valve 68. The pressurerelief valve 68 is suitably adjusted to allow hydraulic liquid to passthrough it at the pressure levels arising in the situation describedabove. The hydraulic liquid entering circuit 50 via port G is collectedin the accumulator.

When the steering system, and therefore the positioning system 40, hasreached the fully locked state, the rear axle 18 still abuts against thetravel stop 38 but the excess hydraulic pressure has now been dissipatedthrough the pressure relief valve 68 associated with port G. Thus, whenthe steering system is taken out of the fully locked state (in this caseby an anti-clockwise turn of the vehicle 10) the slave piston 44 nowretracts correctly as the master piston 48 extends. Moreover, the slavepiston 44 will extend correctly the next time the master piston 48retracts. Hence, the positioning system 40 has been re-aligned to normalalignment.

Similarly, by putting the steering system, and therefore the positioningsystem 40, into the opposite fully locked position (not illustrated) amisalignment caused by an over-extension of the other slave piston 42can be corrected. In this case, the slave piston 42 abuts against theother travel stop 36 and the excess hydraulic pressure is vented throughport F and the associated pressure relief valve 68.

Thus, it will be appreciated that the positioning system 40 allows amisalignment of a steering system of the type described above to becorrected simply by putting the steering system, and therefore thepositioning system 40, into one fully locked state and, if necessary,then the opposite fully locked state.

Ports D and E operate in complement to ports G and F respectively innormal manner when the rams are double acting rams. For example, in theFIG. 2 arrangement, an inflow of oil at port G may be balanced in normalmanner by an outflow of oil at port E. In alternative embodiments (notillustrated), the ports D, E, F, G and associated pressure relief valves68 may operate in alternative configurations. For example, withreference to FIG. 2, if one or more travel stop was arranged to prevent,say, piston 44 of slave ram 24 from retracting beyond a given state,then the build up of excess hydraulic pressure when the positioningsystem was misaligned would be vented mainly by hydraulic liquid passingthrough the pressure relief valve 68 associated with port E.

It will be understood that the slave rams 22, 24 need not necessary bearranged to actuate a rear axle—they could be used to operate anysteerable axle. Further, in cases where the wheels themselves aresteerable, the slave rams may be arranged to actuate the steerablewheels (rather than the axle). Moreover, there need not necessarily betwo pairs of master/slave rams—the invention may be applied inpositioning systems comprising one or more pairs of master/slave rams.The rams need not necessarily be hydraulic but may alternatively beoperatable by any other suitable operating fluid.

The positioning system of the invention is not limited to use in asteering system of an articulated, or any other, vehicle. The inventionmay be employed in any positioning system comprising at least one pairof master/slave rams in order to correct the relative alignment of themaster and slave rams. Depending on the requirements of the application,the, or each, travel stop may be arranged to prevent the, or each,respective slave piston from extending and/or retracting more than isnecessary when the system is normally aligned.

The invention is not limited to the embodiments described herein whichmay be modified or varied without departing from the scope of theinvention.

1. A positioning system comprising at least one master ram andrespective slave ram, each ram being fluid operable and having arespective piston and a respective piston chamber, the or each masterram and respective slave ram being arranged, when in a state of normalalignment, so that actuation of the master piston with respect to themaster piston chamber causes a corresponding actuation of the slavepiston with respect to the slave piston chamber, wherein the systemfurther includes at least one travel stop and at least one pressurerelief valve, the or each travel stop being arranged to restrict thetravel of one or more slave piston when the system is misaligned, atleast one pressure relief valve being arranged to vent excess pressurein the operating fluid arising as a result of such restriction.
 2. Apositioning system as claimed in claim 1, in which, in the state ofnormal alignment, the or each master and respective slave pistons areactuatable between a respective fully extended state and a respectivefully retracted state, wherein the or each travel stop is arranged toprevent one or more slave piston from reaching at least one of saidfully extended or fully retracted states when the system is misaligned.3. A positioning system as claimed in claim 1 or 2, in which each ramcomprises a first port and a second port for the inlet and outlet ofoperating fluid, wherein the ports of the master ram are connected tothe ports of the slave ram with a one-to-one correspondence, and whereina respective pressure relief valve is connected to at least one pair ofconnected ports.
 4. A positioning system as claimed in claim 3, in whicha respective pressure relief valve is connected to each pair ofconnected ports.
 5. A positioning system as claimed in any precedingclaim, wherein the, or each, pressure relief valve is arranged to directvented operating fluid to an accumulator.
 6. A steering systemcomprising a positioning system as claimed in claim
 1. 7. A steeringsystem as claimed in claim 6, wherein the or each master ram ismountable on a vehicle such that, upon turning of the vehicle, the oreach master piston is actuated, and wherein the or each respective slavepiston is adapted for steering at least one wheel of the vehicle.
 8. Asteering system as claimed in claim 7, wherein said at least one wheelis carried by a steerable axle, the or each respective slave pistonbeing adapted to steer said steerable axle.
 9. A steering system asclaimed in claim 7 or 8, arranged for steering an articulated vehiclehaving a tractor unit pivotably coupled to a trailer unit, wherein theor each master piston is actuated upon relative pivoting movementbetween the tractor unit and the trailer.
 10. A steering system asclaimed in claim 9, wherein the arrangement is such that, upon extensionor retraction of the or each master piston, the respective slave pistonis retracted or extended respectively by a corresponding amount.
 11. Asteering system as claimed in any of claims 8 to 10, comprising a firstmaster ram and a second master ram, each associated with a respectiveslave ram, the respective slave pistons being coupled to a steerableaxle of the vehicle at locations on opposite sides of the pivoting axisof the steerable axle, the arrangement being such that, upon turning ofthe vehicle, one of the first and second master pistons is caused toretract, the other being caused to extend, the corresponding actuationof the respective slave pistons causing the steerable axle to pivotabout its pivoting axis.
 12. A steering system as claimed in any ofclaims 9 to 11 when dependent on claim 8, wherein the or each travelstop is provided on the chassis of a vehicle on which, in use, thesteering system in mounted and arranged to restrict the pivotingmovement of the steerable axle.
 13. A vehicle comprising a steeringsystem as claimed in claim 6.